1. Field of the Invention
The present invention relates to a magnetic recording medium and relates to, in particular, a coated type magnetic recording medium on which a magnetic layer is coated.
2. Description of the Related Art
Recently, a high-density, long-time recording is demanded with respect to a magnetic recording medium in which a magnetic layer having magnetic fine powder dispersed in a binder is coated on a non-magnetic substrate.
To realize the high-density recording, it is required to smooth the surface of the magnetic layer to thereby increase carrier output and to decrease noise. To realize the long-time recording, it is required to make a tape as the substrate thinner.
Further, as the mass consumption of the magnetic recording medium as media spreads further, it is required to supply cheaper ones.
Specifically, to provide the high-density recording, it is necessary to enhance the carrier to noise (C/N) ratio of the magnetic recording medium. To this end, it is preferable to smooth the surface of the magnetic layer so as to increase a contact area. On the contrary, the smoothing causes the coefficients of friction between the magnetic layer and the head, between the magnetic layer and the head cylinder and between the magnetic layer and the guide to increase. As a result, the magnetic layer tends to be missed and damaged, the head tends to be contaminated and clogged up or the like.
To prevent the clogging and contamination of the head, conventionally, abrasive material having Mohs' hardness of 8 or more are added to the magnetic layer to abrade the head. However, if the abrasive material are added to the extent that the contamination of the head is prevented, the amount of the abrasion loss (referred to as `abrasion loss` hereinafter) of the head increases to thereby shorten the service life of a video tape recorder (VTR). Also, if the amount of abrasive material increases, the filling ratio of the magnetic powder decreases and the carrier output of the magnetic layer lowers.
In addition, to reduce the frictional resistance of the tape itself, a lubricant such as a carbon black, organic substance or the like is added to the magnetic layer. This, however, tends to contaminate the head further, decrease the strength of the magnetic coating film or the like. Particularly, this is often prone to inflict, for example, damage on the tape edge of a magnetic tape as the magnetic recording medium which is made thinner to allow long-time recording.
To take these drawbacks into consideration, in the magnetic recording medium required to be smoother and thinner, there has been presently provided a back coating layer including a carbon black as a main component which is coated on the surface of the non-magnetic substrate opposite to that on which the magnetic layer is coated. By doing so, it is intended to ensure running characteristics and, at the same time, attain the smoothing of the surface of the magnetic recording medium and the necessary strength of its coating film.
Nevertheless, even if the back coating layer of this type is provided, the contamination of the head resulting from the smoothing of the surface of the magnetic layer is not completely eliminated in actual.
To overcome these problems, Japanese Patent Application Laid-Open Publication Nos. 61-57036 and 60-93631 disclose magnetic recording media in which the number of abrasive material per the surface of the magnetic layer is specified.
However, they cannot completely avoid the contamination of the head, and decrease its abrasion loss and the running resistance of the magnetic recording medium.
Moreover, Japanese Patent Application Laid-Open Publication No. 6-52541 discloses a magnetic recording medium in which the average height of protrusions of abrasive material is specified to 15 nm or less.
If the average height of the abrasive material is specified to 15 nm, the abrasion ability to the head increases and the contamination of the head can thereby decrease. At the same time, however, the abrasion loss of the head increases accordingly. Besides, while it is possible to clean the sliding portion of the head on which the head is already reaching in its clogged-up state by using another magnetic recording medium, the contamination in the head gap cannot be removed. Additionally, even if the magnetic layer is smoothed, the running resistance of the magnetic recording medium cannot be lowered fully. Without applying a back coating layer, it is hard to ensure the sufficient running durability of the magnetic recording medium.